1. Field of the Invention
The present invention is related to methods of forming dielectrics of the type used in semiconductor VLSI applications, and more particularly to improved processes for forming low-dielectric constant fluorinated silicon glass layers having improved characteristics.
2. Description of the Related Art
Capacitive coupling between metal features in an integrated circuit increases in inverse proportion to the distance between the metal features. As the typical metal feature size in integrated circuits decreases with each new generation of circuits, the spacing between metal features in the circuits also decreases. Consequently, as integrated circuits increase in complexity and shrink in size, capacitive coupling between metal features increases in magnitude. The resistance-capacitance (RC) signal delays associated with capacitive coupling similarly grow in magnitude, and degrade the performance of the circuits.
Since capacitance is directly proportional to the dielectric constant (k), RC problems in integrated circuits can be reduced if low dielectric-constant materials are used as the insulating material. Silicon dioxide (SiO2) has long been used as a dielectric for integrated circuits because of its excellent mechanical and thermal stability and relatively good dielectric properties (kxcx9c4.0). However, industry now requires materials with dielectric constants less than that of silicon dioxide for use as intermetal and interlevel dielectrics in modern integrated circuits.
Fluorosilicate glass (FSG) has been identified as a very promising low dielectric constant material for use in integrated circuits. Incorporation of fluorine, a very electronegative atom, into a silicon oxide layer decreases the polarizability and hence the dielectric constant of the layer. Further increasing the fluorine content of the silicon oxide layer generally further decreases the dielectric constant.
Fluorosilicate glass dielectric layers in integrated circuits are simply deposited using chemical vapor deposition (CVD) and plasma enhanced chemical vapor deposition (PECVD) processes as disclosed, for example, in U.S. Pat. Nos. 5,563,105 and 5,827,785. Typically, a mixture of reactant such as tetraethoxysilane (TEOS), SiF4, SiH4, F2, N2O, and O2is introduced into a CVD chamber and excited by a radio frequency (RF) electric field to create ions or radicals which recombine on the substrate surfaces to give the desired FSG film.
Typically, conventional process recipes for the deposition of FSG films, including RF power, choice of reactant gas concentrations, and total pressure, were initially chosen as variations on the baseline process recipes for deposition of silicon oxide films and then improved by experimentation. For example, in some conventional process recipes for PECVD FSG film deposition the total pressure in the CVD chamber was initially chosen to be xcx9c2.4 torr to match a recipe for PECVD silicon oxide film deposition, and then raised to xcx9c2.7 torr to improve the film deposition rate and the film uniformity and to decrease the dielectric constant.
Unfortunately, conventionally deposited FSG films are typically mechanically, chemically, and thermally unstable. For example, loosely bound fluorine atoms in the lattice structure of some FSG films result in the films having a tendency to absorb water. The absorbed water increases the dielectric constant of the film and reacts with fluorine to form corrosive HF. The HF and absorbed water typically outgas during thermal processes, such as anneal processes, and degrade the adhesion properties of the FSG film. For example, outgassing HF and water can cause blister and bubble formation in subsequently deposited metal or dielectric layers.
Some conventionally deposited unstable FSG films can be stabilized by annealing or by other post-deposition treatments. However, such methods require additional process steps, equipment, and expense.
For these reasons, what is needed is an improved process for depositing a robust FSG film on a substrate such that the FSG film exhibits, for example, improved chemical, mechanical, and thermal stability without additional processing.
An improved process for depositing a robust fluorosilicate glass film on a substrate in a chamber includes maintaining a total pressure in the chamber of less than about 1.7 torr, introducing vapor phase chemicals into the chamber, and reacting the vapor-phase chemicals with sufficiently supplied energy to deposit a thin film layer of the fluorosilicate glass on the substrate. In one embodiment, the total pressure in the chamber is about 0.5 torr to about 1.7 torr.
In one embodiment, the vapor-phase chemicals are N2, SiF4, SiH4, and N2O introduced into the chamber at flow rates, in standard cubic centimeters per minute (sccm), with a ratio of about 1.7:0.5:1:7 to about 17:7:1:70. The energy is supplied as RF energy of frequency about 13.56 MHz at a power density of about 0.4 W/cm2 to about 5 W/cm2, and as RF energy of frequency of about 250 kHz and power density about 0.2 W/cm2 to about 3 W/cm2.
Advantageously, fluorosilicate glass films deposited with the improved process are chemically, mechanically, and thermally stable without additional processing. Also advantageously, the films are deposited uniformly at rates greater than about 5000 Angstroms per minute with dielectric constants of about 3.4 to about 3.9.